Efforts are being made worldwide to develop a hydrophone having a high sensitivity which is influenced little (or not at all) by static pressure, i.e. the depth of the water in which the measurements are performed.
A hydrophone wherein use is made of an optical fibre is for instance known from the U.S. Pat. No. 5,247,490 and in European patent application No. 0,554,085.
In practice a glass fibre is arranged on a mandrel-like or disc-shaped support member, wherein pressure variations are measured by the deformation of the support member relative to a reference sensor element which undergoes no mechanical deformation, and wherein the change in length is measured by means of interference between light-beams transmitted through the fibres.
In order to compensate for the static pressure it is for instance known from the European patent application No. 0,554,085 to arrange a compensation chamber beneath a flexible disc-shaped membrane on which the optical sensor is arranged, which compensation chamber is then in open communication via an aperture of predetermined dimensions with the liquid medium in which measurement is taking place. The size of the aperture determines the dynamic behaviour of such sensors.
Such a sensor displays a drastic reduction in sensitivity relative to a sensor with a closed (air) chamber. Increasing the dimensions of the sensor, for instance by a factor of 10 to 20, in order to limit this reduction meets with practical drawbacks during use. In order to obtain sufficient resolution (i.e. the number of measurement points) the smallest possible dimensions are desirable.
The '377 patent discloses a hydrophone wherein a first chamber with a sensor fibre is filled with air and enclosed by water, and a second double-walled cylinder a with reference fibre is filled with air or a gas and enclosed by water. Under hydrostatic pressure both cylinders are deformed and the respective length of each of the fibres is thus changed.